Deadlines have passed and budget guidelines have become hazy memories, but the political castor oil that congressional agriculture committees have tried for months to avoid appears about to become inevitable.

The issue is the writing of a 1985 farm bill, a complicated and increasingly painful exercise that has put the House and Senate committees in a classical bind -- finding a way to cut federal program costs, yet protecting income in the battered farm sector.

Day after day since May, to little avail, members of the two panels have argued, cajoled, pleaded, begged and even postured over the key price-support sections of a bill.

And almost each day as the debate goes on, delegations of sunburned, cap-wearing farmers have trooped into the hearing rooms to sit stoically and watch their fate being forged.

The farmers were there again yesterday as the House panel gritted its teeth and edged closer to the castor oil.

Committee members rejected three farm-support proposals depicted by sponsors as the easiest way out of the bind.

The principal vote came on an amendment by Rep. Arlan Stangeland (R-Minn.) that would have taken farm supports in a new direction through a marketing loan aimed at cutting costs and moving U.S. grain into world markets while holding up farm income.

The plan, which had bipartisan support, would have subsidized farmers by paying them the difference between world prices and predetermined crop loan rates. The proposal lost 22 to 20.

"People around here get very nervous when you start talking about radically changing the structure of the farm-support programs," one farm lobbyist said. "And that is what this was about -- a radical change in the structure."

Stangeland's defeat left one major price-support proposal pending, a plan by Reps. Thomas S. Foley (D-Wash.) and Ron Marlenee (R-Mont.) that is expected to go to a vote today.

It would cut support loan levels, freeze direct subsidy-payments and order a massive slash in wheat and corn plantings to bring supply more in line with demand.

Foley and Marlenee said yesterday that their amendment to the farm bill, similar to a plan pending in the Senate, would hold farm income at least at current levels, increase U.S. export competitiveness and save about $3.6 billion over the three-year budget "baseline" for agriculture.

Although not vastly different from Stangeland's plan, Foley-Marlenee contains a subtle twist. It would require the administration -- rather than Congress -- to make the potentially difficult political decision to move support loan rates down to the market-clearing levels it says are needed to prop up the farm economy.

While not supporting the proposal directly, Reagan administration officials indicated that they could accept the Foley-Marlenee plan as a way of reaching their goal of cutting federal outlays on farm programs. But Richard W. Goldberg, deputy undersecretary of agriculture, indicated budget "concerns" about the plan.

Rep. Dan Glickman (D-Kan.), a Stangeland ally, underlined a central problem in the debate on both sides of Capitol Hill -- the unreliability, and perhaps even meaninglessness, of budget projections and guidelines.

"These numbers are illusory at best," he said. "We are spouting numbers like we were in an arithmetic class."

And that, of course, is key to the bigger dilemma.

As the farm economy has continued to sag, the legislators are under increasing pressure to discard the guidelines and produce a politically palatable farm package for constituents.

Marlenee touched on that yesterday, with a warning. He said that a budget-busting farm bill would not survive on the House floor or in the White House. And, in a jab at other proposals, he cautioned that any sharp program cuts would enrage farmers.

The tension spawned by that reality has sent temperatures rising in the Senate committee, where Majority Leader Robert J. Dole (R-Kan.) is pressuring members to complete work on a bill before they adjourn for the August recess.

Democrats have murmured increasingly in recent days that they will not be pressured into a rush job just to satisfy Dole.

The testiness reached an unprecedented level last week when Chairman Jesse Helms (R-N.C.) took offense at a remark by Sen. Tom Harkin (D-Iowa) about who the "real" chairman of the committee was. Helms told the senator to "watch his tongue." Harkin said it was only a joke, but it was like the farm bill -- not fun at all.

In a related action yesterday, the House approved a $33.1 billion agriculture money bill for fiscal 1986 that basically maintains services at existing levels.

The only floor addition to the bill, which now goes to the Senate, was $2 million for the government to develop and improve tests for acquired immune deficiency syndrome (AIDS).